Trip reports, photos and videos from Rockhounds' list members.
Pete’s Christmas Rock Quiz
1. Arkosic sandstone/comglomerate; call it a fine-grained, angular conglomerate or a coarse-grained sandstone; a sedimentary rock. An arkose is a sandstone that contains feldspar fragments. Not sure exactly where or which rock formation it’s from; somewhere in the foothills near Denver.
2. Shale; not certain where it’s from or its age; somewhere in Colorado (hey, it just happened to be there in the picture). It fizzes slightly in acid, hence, it’s a carbonate-containing shale (“limey” or “calcareous” shale). This may be from the Creede Formation.
3. Granite pegmatite, with some amazonite visible; Pikes Peak region, Colorado.
4. A toughie; I didn’t necessarily expect anyone to guess this one. It’s kimberlite—a fragmental igneous rock (one could call it a type of volcanic breccia). containing light-colored fragments of various rock types, in a dark-colored matrix of serpentine (from alteration of olivine) and other minerals. From Larimer County, Colorado; the Colorado-Wyoming State Line kimberlite district.
5. Microcline feldspar, from one of the pegmatites in Colorado (not at all sure which one).
6. This one I wondered if someone would guess correctly: calcite crystals within a septarian nodule; from the Cretaceous shale in southeast Colorado—Las Animas and Otero counties area. (Barite crystals are also found in these concretions.)
1. Agate (thunderegg), St. Egidien, Saxony, Germany.
2. Orthoclase feldspar crystal (Carlsbad twin), in volcanic rock matrix; Jarilla Mountains, Otero County, New Mexico [agreed, I couldn’t expect anyone to know where this crystal was from, unless by a lucky guess]
3. Salt minerals! Halite (dark blue) and sylvite (white), from Carlsbad, New Mexico. I purchased this specimen at the NM Mineral Symposium, Socorro NM, in November.
4. “Moqui Marble” (iron oxide concretion), southeast Utah.
5. Stibnite, Kapnik (Kapnic, Cavnic), Romania [the specimen was labeled “Antimonit”, the German name for the mineral, when I bought it from David Bunk Minerals in December; from the J. Höppner collection.]
6. Ruby (corundum), Arendal, Norway. [Also from the Höppner collection, as was the agate above, and the amber below.] This crystal seems (?) to have a slightly more purplish tint than most Indian rubies, though I’m not sure about that.
7. Amber, from Germany: “Alter Tagebau (Braunkohle), Lohsa, Lausitz” [= old open pit mine; brown coal = lignite; an area in easternmost Germany]. For the folks from the list who like fluorescence: this amber fl. yellowish-white LW, very weak fluorescence SW. (It’s hard to tell how the apparent fluorescence color is affected by the yellow color of the amber itself.)
8. Pipestone (catlinite), Pipestone, Minnesota (Pipestone National Monument or vicinity). A type of argillite; weakly metamorphosed mudstone or siltstone.